Seismic hazard in the Puget Lowland, Washington state, USA

Boulder, Colo., USA - Seismic hazards in the Puget Lowland of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone and shallow earthquakes associated with crustal faults across the region. Research presented in Geosphere this month establishes not only that one of the more prominent crustal faults, the Darrington-Devils Mountain fault zone, displays evidence of strong earthquakes in the past, but that it will likely be a source of strong earthquakes in the future.

Paleoseismic investigations on the Darrington-Devils Mountain fault zone by Stephen F. Personius and colleagues, using three-dimensional trenching, document a large-magnitude (M 6.7 to 7.0) earthquake about 2,000 years ago and show evidence of a similar earthquake about 8,000 years ago.

An additional surprising result is evidence indicating that the sense of slip on the fault zone during these earthquakes was primarily right-lateral, with a smaller component of north-side-up vertical slip. Holocene north-side-up, right-lateral oblique slip is opposite the south-side-up, left-lateral oblique sense of slip inferred from deformation of Eocene and older rocks along the fault zone. According Personius and colleagues, the cause of this slip reversal is unknown, but may be related to ongoing clockwise rotation of northwestern Washington State into a position more favorable to right-lateral slip in the current stress field.

New findings about the geologic history of the Colorado River and the Grand Canyon;

Time scales of Bald Mountain batholith growth; and

Geometry of the subduction zone along the southwestern coast of Alaska.

All Geosphere articles available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of Geosphere articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geosphere in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

The geologic history of the Colorado River in the Grand Canyon is of keen and lasting interest to geologists and laypersons alike, the problem being that even after 150 years of research that history is not well understood. This paper tests the proposition that the Colorado River once flowed into the Virgin River depression north of the Grand Wash trough, where the river now exits the Grand Canyon. The ages of detrital zircons in the Muddy Creek Formation of the Virgin River depression conclusively refute that hypothesis for Pliocene time, and probably for Miocene time as well. The negative evidence developed in the paper can help guide further investigations of Grand Canyon geologic history.

The Bald Mountain batholith in northeastern Oregon is a classic example of a composite, incrementally constructed batholith that formed during terrane amalgamation outboard of the western U.S. Cordillera. This manuscript investigates time scales and processes of Cordilleran batholith growth during and following lateral terrane collisions. Results from this study document a transient pulse of deep-crustal magmatism from 141 to 147 million years ago that was triggered by partial melting of tectonically thickened island-arc crust. These insights highlight temporally evolving processes of magmatism, crustal thickening and the geochemical maturation of arc crust over a 15-million-year interval along the western U.S. Cordillera.

The nature of the tectonic setting of southeast Alaska in the region of the Yakutat Block microplate has been ambiguous. Geologic features associated with the subduction zone found along the southwestern coast of Alaska, such as the Aleutian Trench and Wadati-Benioff zone, do not extend past the western boundary of the Yakutat Block located near of Prince William Sound. This paper presents the geometry of the subsurface along the boundary between the Yakutat Block and North America produced by passive seismic imaging. We conclude the following: (a) the Yakutat Block is experiencing flat slab subduction under North America; (b) extends to the north at least as far as the Wrangell Mountains; (c) extends to the east as far as the Fairweather Fault; (d) provides the source of magma for the Wrangell Mountains; and (e) forms a continuous surface with the adjacent Pacific Plate.

The Gonghe Basin complex occupies an important position interior to the northeastern Tibetan plateau, at the boundary between the physiographic Tibetan plateau to the south and the narrow, steep-sided ranges of the Qilian Shan to the north. The basin is overthrust along its northern and southern margins by south-vergent thrust faults, and this tectonism has constructed elongate mountain ranges around the perimeter of the basin. In this paper, we review evidence that indicates that these thrust faults have been active since six to 10 million years ago. We present several observations, ranging from structural geology, to geomorphology, to thermochronology, all of which reveal the style and rates of deformation/contraction across the faults/ranges. For the northern range, we also present an estimate of the deformation/contraction rate measured over an approx. 100,000-year time scale, and we find that rates of contraction appear to be somewhat steady through time. We argue that, collectively, our observations show that the ranges in this part of interior northeastern Tibet have been built by listric thrust faults that extend into the middle part of Earth's crust. Moreover, the history of the ranges along the margins of Gonghe basin is similar to the tectonic histories of faults/ranges in the northern Qilian Shan at the plateau margin. The similarity suggests that the entire Qilian Shan region has experienced coordinated/linked deformation since the middle of the Miocene.

Dissected caldera structures expose thick pyroclastic deposits and, uncommonly, cogenetic granite bodies, while related pyroclastic flow material deposited on the pre-eruption ground surface record elements of ancient landscapes. The Middle Fork caldera encompasses a 10 by 20 km area of rhyolite welded tuff and granite in east-central Alaska about 100 km west of the Yukon border. Uranium/lead dating of zircon shows that the rhyolite tuff and granite are 70 million years old (Late Cretaceous). The Middle Fork is a relatively well preserved caldera structure within a broad region of Paleozoic metamorphic rocks and Mesozoic granitic rocks bounded by northeast-trending faults. In the relatively down-dropped and less deeply exhumed crustal blocks, Cretaceous-Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape. The 70-million-year-old land surface is preserved beneath rhyolite tuff at the west margin of the caldera structure and beneath related tuff 16 to 23 km east-southeast of the caldera in the valley of Gold Creek. The tuff in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modified in the past 70 million years other than entrenchment of tributaries in response to lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet about 2.7 million years ago.

From the abstract: The trachydacite complex of Mammoth Mountain and an array of contemporaneous mafic volcanoes in its periphery together form a discrete late Pleistocene magmatic system that is thermally and compositionally independent of the adjacent subalkaline Long Valley system (California, USA). The Mammoth system first erupted about 230,000 years ago and last erupted about 8,000 years ago. and remains restless and potentially active.

From the abstract: Central Oregon (northwestern USA), where northern Basin and Range extension diminishes in magnitude across the High Lava Plains, exhibits widespread extensional faulting and Quaternary volcanism, yet the relations between the processes are complex and chronology is poorly constrained. Here we use cosmogenic 3He exposure dating of basalt lava flows to quantify the timing of normal faulting and emplacement of a lava field on the margins of pluvial Fort Rock Lake.

From the abstract: We use broadband seismic data acquired by the St. Elias Erosion/Tectonics Project (STEEP) and the Alaska Earthquake Information Center to image the geometry of the subducting Yakutat Block in southeast Alaska. We combine results for both P- and S-wave receiver functions. P-wave to S-wave data were imaged with a fully three-dimensional wavefield imaging method centered on the STEEP region.

Sensitivity of uplift patterns to dip of the San Andreas fault in the Coachella Valley, CaliforniaLaura A. Fattaruso et al., Dept. of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA. Published online 12 Nov. 2014; http://dx.doi.org/10.1130/GES01050.1.

From the abstract: Three-dimensional mechanical simulations of the San Andreas fault system within the Coachella Valley in southern California produce deformation that matches geologic observations and demonstrate the first-order impact of fault geometry on uplift patterns. To date, most models that include the Coachella Valley segment of the San Andreas fault have assumed a vertical orientation for this fault, but recent studies of seismicity and geodetically observed strain suggest that this segment of the fault may dip 60 to 70 degrees to the northeast. We compare models with varied geometry along this segment of the fault and evaluate how well they reproduce observed uplift patterns in the Mecca Hills and Coachella Valley.

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